DC Voltage Droop Control Design for MMC-Based Multiterminal HVDC Grids

This article addresses the design of the DC voltage droop control in modular multilevel converter (MMC)-based multiterminal HVDC grids. First, two energy-based control approaches, namely classic and cross control, are explored for the implementation of the voltage-power droop controller. The cross control, as the better solution for droop implementation, is further improved, making it more robust against disturbances. Then, a methodology is derived to select the droop gain combinations considering the AC grid, DC grid and MMC dynamics and their limitations. The methodology is based on a linear analysis to identify the valid droop gains which comply with the limits imposed on: the transient power sharing among MMCs, the DC grid voltage, the MMC AC and DC currents, the total MMC stored energy, and the stability margin of the complete multiterminal HVDC grid. Finally, time-domain simulations are conducted using the nonlinear model to validate the dynamic performance of the selected droop combinations obtained from the suggested methodology.